It is known that polypeptides can be expressed in a wide variety of cellular hosts. A wide variety of structural genes have been isolated from mammals and viruses, joined to transcriptional and translational initiation and termination regulatory signals from a source other than the structural gene, and introduced into hosts into which these regulatory signals are functional.
For economic reasons, it would be desirable to utilize genetically engineered unicellular microorganisms to produce a wide variety of polypeptides. However, because of the inherent differences in the nature of unicellular organisms on one hand and mammalian cells on the other, the folding and processing of polypeptides in unicellular microorganisms appears to be quite different from the folding and processing that is effected in mammalian cells. As a result, mammalian polypeptides derived from unicellular microorganisms are not always properly folded or processed to provide the desired degree of biological or physiological activity in the obtained polypeptide.
To that end attempts have been made, with varying degrees of success, to express mammalian polypeptides in plants. One particularly important polypeptide is secretory immunoglobulin A.
Secretory immunoglobulin A (SIgA) is the most abundant form of immunoglobulin (Ig) in mucosal secretions, where it forms part of the first line of defense against infectious agents. The molecule exists mainly in the 11S dimeric form, in which two monomeric IgA antibody units are associated with the small polypeptide joining (J) chain and with a fourth polypeptide, secretory component (SC). The ability to produce monoclonal SIgA would be of substantial value, but the synthesis is complicated because it requires plasma cells secreting dimeric IgA (dIgA) as well as epithelial cells expressing the polymeric Ig receptor (pIgR). Normally, pIgR on the epithelial basolateral surface binds dIgA, initiating a process of endocytosis, transcytosis, phosphorylation, proteolysis, and ultimate release of the SIgA complex at the apical surface into the secretion (Mostov, Ann. Rev. Immunol. 12: 63 (1994)). Thus, it is important to focus on the ability of transgenic plants to assemble secretory antibodies.
Secretory IgA is resistant to denaturation caused by harsh environments. This denaturation resistance requires that the complex secretory IgA molecule containing IgA molecules, J chain and secretory component be accurately and efficiently assembled. Until the present invention, assembly and expression of useful amounts of secretory IgA was impractical, due to low yields and due to the inability of the available mammalian systems to express and assemble SigA in a single cell. As disclosed herein, the foregoing problems have now been solved by the present invention.
The expression of a multimeric protein in plant cells requires that the genes coding for the polypeptide chains be present in the same plant cell. Until the advent of the procedures disclosed herein, the probability of actually introducing both genes into the same cell was extremely remote. Assembly of multimeric protein and expression of significant amounts of same has now been made feasible by use of the methods and constructs described herein.
Transgenic plants are emerging as an important system for the expression of many recombinant proteins, especially those intended for therapeutic purposes. One of their major attractions is the potential for protein production on an agricultural scale at an extremely competitive cost, but there are also many other advantages. Most plant transformation techniques result in the stable integration of the foreign DNA into the plant genome, so genetic recombination by crossing of transgenic plants is a simple method for introducing new genes and accumulating multiple genes into plants. Furthermore, the processing and assembly of recombinant proteins in plants may also complement that in mammalian cells, which may be an advantage over the more commonly used microbial expression systems.
One of the most useful aspects of using a recombinant expression system for antibody production is the ease with which the antibody can be tailored by molecular engineering. This allows the production of antibody fragments and single-chain molecules, as well as the manipulation of full-length antibodies. For example, a side range of functional recombinant-antibody fragments, such as Fab, F.sub.v, single-chain and single-domain antibodies, may be generated. In addition, the ability of plant cells to produce full-length antibodies can be exploited for the production of antibody molecules with altered Fc-mediated properties. This is facilitated by the domain structure of immunoglobulin chains, which allows individual domains to be "cut and spliced" at the gene level. For example, the C-terminals domains of an IgG antibody heavy chain have been modified by replacing the C.gamma.2 and C.gamma.3 domains with C.alpha.2 and C.alpha.3 domains of an IgA antibody, while maintaining the correct assembly of the functional antibody in plants. These alterations have no effect on antigen binding or specificity, but may modify the protective functions of the antibody that are mediated through the Fc region.
It is also becoming more clear that specially engineered plants may provide an excellent source of various proteins, including therapeutic immunoglobulins, in large quantities and at a relatively low cost. Production of antibodies in plants may be of particular benefit in the area of topical and preventive immunotherapy.
For example, topically applied antibodies can prevent colonization by pathogenic bacteria, as well as modify the resident bacterial flora in a highly specific manner. In the case of dental caries, topically-applied monoclonal antibodies raised against the cell-surface adhesin of Streptococcus mutans prevents the bacteria from becoming established in non-human primates, and also reduces the level of disease (Lehner, et al., Infect. Immun. 50: 796-799 (1985)). In humans, the mAb was shown to confer long-term protection against S. mutans in adults (Ma, et al., Infect. Immun. 50: 3407-14 (1990)).
Thus, methods of providing useful immunoglobulins--particularly antibodies--in large quantities and at low cost confer a distinct advantage over other methodologies in current use. In addition, the relative ease with which one may engineer immunoglobulins and other large protein molecules using a recombinant expression system in plants, and the stability of those systems in succeeding generations, make transgenic plants an extremely attractive source of immunotherapeutic molecules.